1. Field of the Invention
This invention relates to a video display system of the kind in which at least one visible characteristic of consecutive image points on the screen of a raster-scan CRT is defined by the values of consecutive pels of a digital video drive waveform, each such pel comprising one or a plurality of video bits in parallel, and in which a pulse stretching circuit is provided for extending the duration of selected pels in the video waveform in order to at least partially compensate for image distortion introduced by the finite video amplifier rise and fall times of the CRT. A system of this kind is described in IBM TDB, Vol. 24, No. 11B, page 5794 and is used in the IBM 8775 terminal.
2. Description of the Prior Art
The video channel of a high content raster-scan CRT display must operate at a very fast data rate if flicker is to be avoided. For example, a data display having 1.2 million image points refreshed at 60 Hz with a non-interlaced raster requires a peak data rate of about 100 Mpels/Sec. This corresponds to a pel period of 10 nSecs. Full modulation of the electron beam requires a cathode drive voltage of about 35 volts for a monochrome tube and up to 60 volts for color. It is very difficult to design a video amplifier to produce these voltage transitions in a time which is short compared to the pel period. This is particularly true if the amplifier must handle analog signals rather than a simple binary waveform. In this case 10 to 90% rise and fall times of 7 nSecs are considered state-of-the-art for a color display. Such an amplifier will produce greatly distorted video pulses compared to the ideal rectangular shape. For the user the effect is particularly noticeable on vertical strokes which have much reduced contrast if they are only one image point wide. The problem is most severe with a monochrome bright-on-dark display (hereinafter referred to as a white-on-black display for convenience) because the beam current is proportional to the drive voltage raised to a power gamma, where gamma is typically 2.2. Consequently, the contrast of a single image point is effectively related to the drive pulse width measured near the voltage for peak white and this is only a few nSecs for a white pulse with the figures quoted.
One known solution to this problem, used in white-on-black displays and referred to above, is to extend the trailing edge of positive (white) pels by logically OR'ing the video waveform with a delayed version of itself. Obviously this technique lengthens positive pels by shortening negative ones and as such is unsuitable for displays having mixed white-on-black and black-on-white information. This problem can be overcome in the restricted case when all the information in a particular region of the display screen is known by the system to have the same polarity. In this case the video signal can be inverted before and after the basic pulse stretching circuit by two exclusive OR gates fed with a signal indicating the information polarity.
However, this technique has several major drawbacks for highly dense displays. In order to cope with mixed polarity displays of high density the system itself must have knowledge of the polarity of the display in each region of the screen. However, even where the polarity is known a highly dense display would have a significant number of image points of opposite polarity to the main information which are isolated in the raster scan direction, and such points would inevitably be reduced in width by the automatic delay of the trailing edge of the immediately preceding pel. Also, the technique cannot be extended to displays with several bits per pel.